This invention is directed to thermoplastic compositions having an improved balance of melt viscosity and impact strength, and in particular to thermoplastic compositions comprising a copolyestercarbonate copolymer, and articles prepared therefrom.
Polycarbonates are amorphous polymers noted for their optical properties, thermal resistance, toughness, and impact resistance. High flow polycarbonates, polyesters, and other polymers, typically those having low molecular weights and are available for the injection molding of parts with very thin dimensions and long flow lengths.
Copolyestercarbonates, having both ester and carbonate repeat units, are known to have polycarbonate-like properties together with substantially increased flow at the same molecular weight as an ordinary aromatic polycarbonate. EP 0510 958 B1 and U.S. Pat. No. 5,605,980, for examples discloses a composition comprising bisphenol-A-copolyestercarbonates having dodecanedioate content with improved melt flow compared to polycarbonates of the same molecular weights. EP 0510 958 B1 does not disclose examples of compositions comprising polyesters, impact modifiers, fillers, or fluoropolymers.
Although enhanced flow allows more rapid molding operations, as well as molding of large and/or complex articles, common methods to do this, such as lowering molecular weights of one or more polymer components of the composition or the addition of flow promoters, have their limitations, as they generally negatively influence other desired properties, especially impact properties.
It is undesirable for the impact strength of the polycarbonate and other mechanical properties (elongation at break, fatigue stress, and the like) to be compromised. Thin-wall and complex injection molded parts with minimal residual stress and low temperature ductility are therefore not readily prepared from existing polycarbonate materials and polymer blends.
Furthermore, in order to obtain thermoplastic compositions with enhanced modulus/stiffness, commonly reinforcing fillers are added to the composition, such as glass fibers, aramid fibers, carbon fibers, and various mineral fillers. However, in most cases, the addition of these fillers leads to brittle failure of the polymer and to lower impact strength. Additionally, processing problems can occur, due to incompatibility between the polymer matrix and the filler.
WO 2008/063724 discloses blends of a polyester with at least two different copolyester carbonates, which comprise arylate ester units. Blends of polyesters and polycarbonates are also known. An alloy or blend of polyester with polycarbonate can offer some improvements with respect to the properties of polycarbonates. For example, alloys of a polycarbonate, a polyester such as polyalkylene terephthalate, and an emulsion-ABS graft copolymer are disclosed in U.S. Pat. No. 7,067,567. This patent discloses a flame resistant polymer composition containing no chlorine or bromine elements and improved weld line strength for molding.
U.S. Patent Publications 2005/0143508 and 2006/0142455 describe particular polymer compositions that give optimum balance of mechanical properties, especially high modulus combined with good ductility and impact strength. These compositions contain a matrix polymer component and a combination of fluoropolymer and mineral filler. U.S. Patent Publications 2008/0242789 and 2008/0246181 describe specific polymer compositions containing a blend of polyesters and polycarbonates and a combination of mineral filler and fluoropolymer, which again have an optimum balance of impact and modulus.
Another family of well-known polycarbonate copolymers are the polycarbonate-polysiloxane copolymers (see for instance U.S. Pat. Nos. 3,189,662 and 3,419,635). U.S. Pat. No. 6,072,011 describes a specific class of compositions of these copolymers that have both improved processing characteristics and improved low temperature impact properties.
It would be desirable to obtain thermoplastic compositions having a superior balance of properties combined with enhanced flow characteristics. Modification of the molecular weights of the thermoplastic components of these compositions can yield some flow improvements without losing too much impact properties, but further improvements in both melt flow and impact strength are desired.
In view of the above, a need exists for polymer compositions that have improved combinations of properties, such as higher modulus or impact strength, in addition to improved melt flow characteristics. It is especially difficult to obtain high modulus, good melt flow characteristics, and good impact strength, all in a single thermoplastic composition.